Flood risk modelling based on tangible and intangible urban flood damage quantification J. A. E. ten Veldhuis and F. H. L. R. Clemens ABSTRACT J. A. E. ten Veldhuis (corresponding author) F. H. L. R. Clemens Department of Water Management, DeIft University of Technology, Stevinweg 1, 2628 CN, DeIft, The Netherlands E-mail: j.a.e.tenVeldhuis@tudelft.nl The usual way to quantify flood damage is by application stage-damage functions. Urban flood incidents in flat areas mostly result in intangible damages like traffic disturbance and inconvenience for pedestrians caused by pools at building entrances, on sidewalks and parking spaces. Stage-damage functions are not well suited to quantify damage for these floods. This paper presents an alternative method to quantify flood damage that uses data from a municipal call centre. The data cover a period of 10 years and contain detailed information on consequences of urban flood incidents. Call data are linked to individual flood incidents and then assigned to specific damage classes. The results are used to draw risk curves for a range of flood incidents of increasing damage severity. Risk curves for aggregated groups of damage classes show that total flood risk related to traffic disturbance is larger than risk of damage to private properties, which in turn is larger than flood risk related to human health. Risk curves for detailed damage classes show how distinctions can be made between flood risks related to many types of occupational use in urban areas. This information can be used to support prioritisation of actions for flood risk reduction. Since call data directly convey how citizens are affected by urban flood incidents, they provide valuable information that complements flood risk analysis based on hydraulic models. Key words | flood risk, intangible damage, risk curve, urban drainage INTRODUCTION Quantitative flood risk modelling consists of two steps: probability estimation and flood damage quantification. Methods to quantify flood damage for severe floods are usually based on stage-damage functions that quantify damage based on inundation depth, flood duration and occupational land use (Thicken et al. 2005). Such functions focus on damage to buildings and building contents, which constitute the main part of total flood damage for severe floods (Defra 2004). Such floods typically have a low probability of occurrence and affect large areas at once. Urban drainage systems in lowland areas are typically designed to cope with rainfall events with return periods of 2 to 5 years (e.g. RIONED 2004). As a result, urban flood incidents occur on a regular basis. Many of these incidents are characterised by small flood depths and small geographical extension. Stage-damage functions are not applicable to quantify damage for such small flood depths (Dutta et al. 2003; Apel et al. 2004; Merz & Thicken 2005). Damage quantification of small flood incidents is complicated by the mainly intangible nature of the damage. Typical consequences are disruption of road traffic and inconvenience for pedestrians caused by pools in front of shops, on parking lots and sidewalks. A method to translate such consequences into monetary values is by assessing people’s willingness to pay (WTP) to prevent flood consequences. This method was applied in the UK, where WTP to prevent health effects of flooding was investigated in a series of questionnaires (Defra 2004) which resulted in doi: 10.2166/wst.2010.243 189 Q IWA Publishing 2010 Water Science & Technology—WST | 62.1 | 2010